Color laser and LED printers produce images using a combination of separate color planes. The printers include a number of different image forming units that each produce the image in a single color. The final image is formed of a combination of each of the overlapping each of the single color images. By way of example, a four color laser printer includes four separate printheads that each form an image of a single color on an intermediate transport mechanism or a media sheet. As the mechanism or sheet moves through the printer, the first printhead forms an image in yellow, a second in cyan, third unit in magenta, and fourth in black. Each of the single color images are accurately formed in an overlapping arrangement to produce the final multi-color image.
Misalignment occurs when one or more of the printheads is not accurately aligned. Often times, misalignment occurs when the image forming apparatus is initially manufactured. Misalignment may further occur gradually over time as the apparatus is in use. Misalignment of one or more of the colors planes results in poor quality images having poor resolution and coloring. The greater the amount of misalignment, the poorer the image quality.
Various prior art methods of color plane alignment have been used to overcome this problem. One method includes producing targets having two or more colors on the media sheet or transport mechanism. The targets are scanned by a high resolution scanner that precisely registers the positions of each of the color planes. High resolution scanners are often expensive, and require regular maintenance to perform properly.
Another method of color plane alignment includes producing targets on the media sheet or intermediate transfer mechanism and determining the location as the target is moved through a particular point within the image forming unit. In one example, the exact position of a point is determined on the media sheet or intermediate transfer mechanism. The target is then formed on the sheet or transport mechanism at a calculated distance from the exact location. Once the sheet is passed through a sensor, the sensed distance is compared to the calculated distance to determine the amount of misalignment. A drawback of this method is the offset of each color plane is determined relative to a particular point on the media sheet or intermediate transfer mechanism. Color plane offset is not determined relative to the other color planes.
The present invention is directed to methods of determining misalignment between color planes within an image forming unit. The methods include forming a first pattern with a first color, and forming a second pattern with a second color. The misalignment is determined by the difference in the amount of the second color within a first section of the first pattern to the amount of the second color within a second section of the first pattern. Misalignment may further be determined by the ratio of the second color amounts within the sections of the first pattern.
In one embodiment, the first pattern formed in a first color includes first and second non-printed sections that are separated by an intermediate section. The first and second sections may have a variety of orientations and dimensions. The second pattern formed in a second color is sized to extend over the intermediate section and into the first and second sections. When there is no misalignment, the amount of the second color is equal within the first and second sections. Increasing amounts of misalignment are determined by increasing differences in the amount of color within the two sections. The amount of misalignment may further be determined by a ratio of the amount of color within the first and second sections.
In one embodiment, the first and second patterns are formed and scanned into a bitmap format. The scanning process may result in fringe effects that distort the amount of color within the first and second sections. To compensate for these effects, the patterns may include a third section sized approximately equal to the first and second sections of the first pattern. The third section may be used for canceling the fringe effects from the first and second sections.